The melatonin analog, iodo-melatonin, may provide us with an ideal agonist to identify characterize, and localize the melatonin receptor. We have made radioactive and "cold" iodomelatonin. This product has been characterized by HPLC and the "cold" iodo- melatonin has been shown to be biologically active. Preliminary data indicate that melatonin can displace iodomelatonin binding to a crude supernatant of hamster brain, but no binding was observed with similarly prepared rat brain homogenate.